This invention relates to battery operated electronic circuits and in particular to those utilizing a rechargeable battery.
The use of batteries as sources of operating voltage in communication equipment, in particular in those devices in which it is desirable to have a high degree of mobility, has become widespread. In addition, consumer electronics products such as television and radio receivers are also available in battery-powered form. Generally radio receivers utilize disposable batteries while television receivers employ rechargeable types. Typically, television receivers include circuitry enabling the user to recharge the batteries during periods of non-use in preparation for subsequent use.
Of the many types of rechargeable batteries available, the most commonly used to operate higher power circuits (such as television receivers) is the lead-acid type which include a plurality of chemical compound bearing plates within an enclosed cavity. The cavity is filled with an electrolyte in either a liquid or "jell" form which serves as a reactant and charge carrier. During the charging process, a reversible chemical reaction stores energy in the battery in the form of a chemical imbalance. Energy is given up during battery discharge by a chemical reaction which essentially reverses the reaction occurring during charging. While these batteries provide a satisfactory power source for higher power circuitry, they are expensive and subject to several operational limitations, the most significant of which is susceptibility to damage due to excessive discharge. There exists in such batteries a minimum charge level discharge below which renders the battery unrechargeable and therefore useless. This creates considerable hazard to battery life since the battery will continue to supply power after the minimum charge level has been reached giving rise to the possibility of inadvertent battery damage.
The use of such power sources to supply a television receiver is further complicated by the varying current requirements thereof. Variations in the load presented by the signal processing and display forming circuitry of a television receiver can vary in excess of 20% due to operator control adjustments. This degree of current variation presents difficulties in the design and application of battery protective circuitry in such applications. Properly designed battery-operated television sets accommodate changes in operating voltage caused by normal battery discharge. Therefore, it is often the case that no receiver malfunction occurs during battery discharge which would serve as an indication to the viewer that the battery is being rendered unrechargeable by continued use.
The use of presently available fuses or thermally activated breakers interposed between the battery and the receiver circuitry will, under most conditions, provide protection from damage caused by high current drain. However, such devices are current responsive and cannot protect the battery discharging below its minimum charge level. In addition, because battery powered consumer electronic devices, for example, television receivers, are operated by persons having little knowledge of their operation, it is desirable to have a positive-response automatic protection circuit which the operator cannot override. However, the cost restrictions inherent in competitive consumer electronics markets makes the use of complex costly protection circuits unlikely.